[ddiomede]

That's very weird. I have well over 200lbs of that same exact sand and I'm not seeing any PH buffering with it. My tap ph is between 7.4 - 7.6, and that's what my degassed tank water sits at. If it was causing my PH to rise, I'd use way more CO2 than I actually am. I've been running the same 10lbs CO2 tank since around Memorial Day so if the controller was up against constantly rising PH I would have exhausted that tank months ago.

I'd ignore the "compatible with salt systems" tag on there because all pool filter sand is compatible with salt water pools/spas. The reason it's compatible with salt water pools/spas is because it's inert and doesn't have any buffering capabilities.

What else do you have in your tank? Rocks, decorations, etc.

What I'd do to eliminate variables is take two water samples....one from your tank and one from the tap. Let both sit out 48 hours and then test them both.

If you want to get really crazy, take a third same with tap and toss some sand in the sample...roughly approximating the sand to tank water ratio and let that sit 48 hours as well.

If those three samples all come out relatively close to one another, then it'll at least give you the ability to eliminate a variable.

But pool filter sand is silica which is PH neutral (7.0). Aragonite is the stuff that buffers PH, which can't be used in saltwater pools/spas because of its PH altering properties. Aragonite is great for saltwater aquariums or African Cichlid tanks but not for much else.

 

[ddiomede]

I'm quite confused by this. I've done some testing since yesterday, so let me list those results along with the original ones and answer some other questions asked above.

• I have Manten stone in the tank, and a piece of black spider wood that had been soaking for about two months. Other than that, only the sand. I also have mechanical/bio media and carbon.
• The pH of the tank when it inspired me to make this post was 8.15, approximately one day after filling it up for the first time. I didn't rinse the sand.
• After posting yesterday, I changed 90% of the water. The pH went back down to ~7.2, which is the right pH for NYC tapwater (correction from where I said it was "dead neutral").
• Then it started climbing slowly throughout the day and topped off at 7.58 before retreating to 7.45 this morning. Still higher than the spec sheet for my tapwater, but not crazy high.
• I took some tank water with sand, some tank water with Manten stone, and some tap water, and let them sit overnight. Tested them just now and they all varied slightly between 7.25 and 7.31, lower than the main tank. I'll test them again tomorrow.

I have few theories. Maybe there was something in the tank or the sand that spiked the pH when I first filled it and the water change got rid of it? It does seem that it isn't the sand after all, but something is at least somewhat increasing my tank pH.

It's an utter mystery why pH was so high yesterday. I might need to recalibrate my meter (calibration chems are coming tomorrow) but not by that much. I was pretty careful calibrating it. I can't throw out the result as a white elephant, it was definitely high.

I didn't know about degassing a water sample. I'm new to planted tanks. So if I understand that correctly, it means to let all the C02 evaporate to get a true pH reading on the water, and that may be higher, due to less C02?

Suggestions welcome!

Degassing just allows the water sample to allow any gases present in the water to dissipate. Some recommend 24 hours, others 48 hours. If I need to test a degassed water sample, I just leave it out for a couple days and set a reminder on my phone for when it's ready to be tested. The only time I've done this is to test my tap water, and my tank water so that I could get baselines for the amount of CO2 I plan on injecting, or if something wonky is going on.

From what I understand Manten stone is Ph neutral. I don't know a whole lot about it, but just recall when I was looking for stone that was one of the ones that didn't mess with the PH.